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First teaching 2020

Last exams 2024

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Internal Resistance (CIE AS Physics)

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Katie M

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Internal Resistance

  • All power supplies have some resistance between their terminals
    • This is called internal resistance (r)

  • This internal resistance causes the charge circulating to dissipate some electrical energy from the power supply itself
    • This is why the cell becomes warm after a period of time

  • The internal resistance therefore causes loss of voltage or energy loss in a power supply
  • A cell can be thought of as a source of e.m.f with an internal resistance connected in series. This is shown in the circuit diagram below:

Internal resistance circuit, downloadable AS & A Level Physics revision notes

Circuit showing the e.m.f and internal resistance of a power supply

 

  • VR is the terminal potential difference
    • This is the voltage available in the circuit itself
    • Terminal p.d = I × R (Ohm’s law)

  • When a load resistor is connected, current flows through the cell and a potential difference develops across the internal resistance. This voltage is not available to the rest of the circuit so is called the ‘lost volts’
  • Vr is the lost volts
    • This is the voltage lost in the cell due to internal resistance, so, from conservation of energy:
    • Lost volts = e.m.f − terminal p.d
    • Lost volts = I × r (Ohm’s law)

  • The e.m.f is the sum of these potential differences, giving the equation below

EMF equation, downloadable AS & A Level Physics revision notes

e.m.f equation

 
  • e.m.f is therefore the total, or maximum, voltage available to the circuit

Worked example

A battery of e.m.f 7.3 V and internal resistance r of 0.3 Ω is connected in series with a resistor of resistance 9.5 Ω.WE - internal resistance question image, downloadable AS & A Level Physics revision notesDetermine:

a)     The current in the circuit

b)     Lost volts from the battery

Worked example internal resistance (2), downloadable AS & A Level Physics revision notes

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Katie M

Author: Katie M

Expertise: Physics

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.